The present invention relates to a light transmissive semiconductor substrate. FIGS. 2(a)-2(d) show a conventional method of forming a Semiconductor-On-Insulator substrate (hereinafter, referred to as "SOI" substrate), utilizing a laminating method of a pair of single crystal silicon plates.
In the FIG. 2(a) step, a single crystal silicon plate 21 is thermally oxidized to form thereon a silicon oxide film 22 (SiO.sub.2). In the FIG. 2(b) step, there is prepared another single crystal silicon plates 23. In the FIG. 2(c) step, the pair of silicon plate 21 and 23 are laminated with each other through the silicon oxide film 22 under a high temperature condition. In the FIG. 2(d) step, the silicon plate 21 formed with the silicon oxide film 22 is thinned to below several microns thickness by polishing or etching. The thus obtained SOI substrate of the conventional structure has a single layer of silicon oxide film interposed between a thick layer of single crystal silicon and a thin layer of single crystal silicon.
FIGS. 3(a)-3(d) show an etching process of the conventional SOI substrate or SOI wafer. In the FIG. 3(a) step, there is prepared a multi-layered structure of a thick silicon plate 33, a silicon oxide film 32 and a thin silicon film 31 in which a device element such as a transistor is to be formed and over which a resist film 34 is coated. In the FIG. 3(b) step, the resist film 34 is selectively removed by exposure and development treatment to form windows. In the FIG. 3(c) step, the silicon film 31 is dry-etched through the windows. The thin silicon film 31 has a variation in thickness over the wafer. In order to completely etch away the silicon film 31 from the respective windows, the silicon oxide film 32 is undercut entirely or partly during the course of the etching process, depending on the local thickness of the overlaying silicon film 31, thereby occasionally forming a deep trench. In the FIG. 3(d) step, the resist film 34 is removed to expose silicon islands 36 bordered by trenches 35. In the case where the depth of the trench is rather large, there are caused various drawbacks in later steps of exposure and development, such as that a focusing adjustment is difficult on the top of the silicon island 36 and the bottom of the deep trench 35 due to a gap therebetween to thereby prevent formation of a precise pattern. Further, in the case where the silicon oxide film 32 is utilized functionally in view of its optically transparent nature, the at least partial removal of the silicon oxide film during the etching process would cause serious defects.
FIGS. 4(a)-4(b) show an example of the specific technology in which a patterned single crystal silicon film is transferred to a transparent substrate by adhesion, and to which one aspect of the present invention is associated. In the FIG. 4(a) step, there is prepared an SOI substrate composed of single crystal silicon islands 41 which are several hundred .ANG. to several times ten .mu.m thick formed with a device element such as a transistor, an underlying silicon oxide film 42 and a single crystal silicon plate 43. These layers 41, 42 and 43 are laminated together in the form of the conventional SOI wafer. A transparent substrate 45 composed of quartz or glass is fixed to the SOI wafer through a transparent adhesive 44. Thereafter, the opaque and thick silicon plate 43 is etched away. After finishing the etching process, the silicon plate 43 is lost such that the silicon islands 41 are transferred to the transparent substrate 45 so that an optically transparent region 46 is formed around the opaque islands 41. That is, light can be transmitted through the transparent regions 46 formed between the silicon islands 41. This type of quartz substrate is utilized to constitute a light valve element in which an optically active material such as a liquid crystal is applied to the transparent region 46.
As an example of a practical construction, the rear silicon plate 43 has a thickness on the order of 500-600 .mu.m, and the silicon oxide film 42 has a thickness in the range of 0.2-1.0 .mu.m. Therefore, as shown in FIG. 4(b), the silicon oxide film 42 may not function efficiently as an etching stopper during the course of etching the rear silicon plate 43, so that the silicon oxide film 42 may be eliminated entirely from the substrate surface. Further, in the progression of the etching process, the rear silicon plate 43 is variably etched due to local differences in an etching rate over the plate material, thereby disadvantageously exposing spots of the silicon oxide film 42 before completely removing the silicon plate 43. At this time, the exposed spot of the silicon oxide film may be broken by stress applied from a surrounding portion of the silicon plate.
When attempting to build a substrate for an optical switching device shown in FIGS. 4(a)-4(b) using a substrate for an integrated circuit shown in FIG. 2(d) via the steps shown in FIGS. 3(a)-3(d), the insulating silicon oxide film of the SOI wafer may be disadvantageously over-etched when etching the thin silicon film 31 (FIG. 3(b)) to form the silicon islands 36 (FIG. 3(c)) on which a device element such as a transistor is to be formed in FIG. 3. Further, as noted in conjunction with FIGS. 4(a) and 4(b), the insulating silicon oxide film of the SOI wafer may be entirely or partly eliminated or may be broken disadvantageously when etching the thick silicon plate 43 or silicon support.